Normally, in an automotive vehicle, the vehicle is steered by turning a steering wheel. When the steering wheel is turned, the vehicle wheels are turned through the action of a steering gear such as a rack and pinion gear set. In a hydraulic, power assist system, a hydraulic motor is coupled to the steering gear set to assist the operator by reducing the manual effort required to steer the vehicle.
A typical hydraulic, power assist, steering system includes a power steering pump, a valve which is actuated upon turning of the steering wheel, and a hydraulic motor controlled by the valve. The hydraulic motor is connected to a steering gear element and provides assistance to the operator's manual steering effort.
The power steering pump is driven by the vehicle engine. The valve is interposed between the pump and the hydraulic motor to control flow of pressurized fluid from the pump to the hydraulic motor. The valve responds to steering maneuvers to control (i) the direction of fluid flow to the steering motor and (ii) the volume of the fluid flow.
Maximum vehicle steering efforts usually occur at zero or low vehicle speeds, such, as during dry surface parking. Power steering pumps are typically designed to deliver sufficient fluid flow and pressure at low engine speeds to the hydraulic motor to insure adequate steering assistance.
Power steering pumps typically provide increasing output flow as pump speed increases up to a predetermined speed, and thereafter provide a substantially constant output flow. Such flow characteristics are achieved by different pump constructions. Typical pump constructions are shown in U.S. Pat. Nos. 4,014,630; 3,817,266; and U.S. patent application Ser. No. 381,702 filed May 25, 1982.
Electronically controlled valves have been developed for use in hydraulic power assist steering systems. Such electronic controls typically have separated the two control functions, one being fluid flow direction control and the other being flow rate control. Fluid flow direction control in such electronically controlled valves is accomplished by the use of a solenoid. Valves that control fluid flow direction are subject to high fluid pressures and consequently have substantial flow forces acting thereon. As a result, the valve solenoid is typically large and has large power requirements.